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1.  Allosteric activation of kinases: Design and application of RapR kinases 
Current Protocols in Cell Biology  2011;CHAPTER:Unit-14.13..
Here we describe a method for the engineered regulation of protein kinases in living cells, the design and application of RapR (rapamycin regulated) kinases. The RapR kinase method enables activation of kinases with high specificity and precise temporal control. Insertion of an engineered allosteric switch, the iFKBP domain, at a structurally conserved position within the kinase catalytic domain makes the modified kinase inactive. Treatment with rapamycin or its non-immunosuppresive analogs triggers interaction with a small FKBP-rapamycin-binding domain (FRB), restoring the activity of the kinase. The reagents used in this method are genetically encoded or membrane permeable, enabling ready application in many systems. Based on the structural similarity of kinase catalytic domains, this method is likely applicable to a wide variety of kinases. Successful regulation has already been demonstrated for three kinases representing both tyrosine and serine/threonine kinase families (p38, FAK, Src). Procedures for designing and testing RapR kinases are discussed.
doi:10.1002/0471143030.cb1413s53
PMCID: PMC3269071  PMID: 22161545
kinase; allosteric; activation; phosphorylation
2.  Large FK506-Binding Proteins Shape the Pharmacology of Rapamycin 
Molecular and Cellular Biology  2013;33(7):1357-1367.
The immunosuppressant and anticancer drug rapamycin works by inducing inhibitory protein complexes with the kinase mTOR, an important regulator of growth and proliferation. The obligatory accessory partner of rapamycin is believed to be FK506-binding protein 12 (FKBP12). Here we show that rapamycin complexes of larger FKBP family members can tightly bind to mTOR and potently inhibit its kinase activity. Cocrystal structures with FKBP51 and FKBP52 reveal the modified molecular binding mode of these alternative ternary complexes in detail. In cellular model systems, FKBP12 can be functionally replaced by larger FKBPs. When the rapamycin dosage is limiting, mTOR inhibition of S6K phosphorylation can be enhanced by FKBP51 overexpression in mammalian cells, whereas FKBP12 is dispensable. FKBP51 could also enable the rapamycin-induced hyperphosphorylation of Akt, which depended on higher FKBP levels than rapamycin-induced inhibition of S6K phosphorylation. These insights provide a mechanistic rationale for preferential mTOR inhibition in specific cell or tissue types by engaging specific FKBP homologs.
doi:10.1128/MCB.00678-12
PMCID: PMC3624267  PMID: 23358420
3.  Biological constraints limit the use of rapamycin-inducible FKBP12-Inp54p for depleting PIP2 in dorsal root ganglia neurons 
Background
Rapamycin-induced translocation systems can be used to manipulate biological processes with precise temporal control. These systems are based on rapamycin-induced dimerization of FK506 Binding Protein 12 (FKBP12) with the FKBP Rapamycin Binding (FRB) domain of mammalian target of rapamycin (mTOR). Here, we sought to adapt a rapamycin-inducible phosphatidylinositol 4,5-bisphosphate (PIP2)-specific phosphatase (Inp54p) system to deplete PIP2 in nociceptive dorsal root ganglia (DRG) neurons.
Results
We genetically targeted membrane-tethered CFP-FRBPLF (a destabilized FRB mutant) to the ubiquitously expressed Rosa26 locus, generating a Rosa26-FRBPLF knockin mouse. In a second knockin mouse line, we targeted Venus-FKBP12-Inp54p to the Calcitonin gene-related peptide-alpha (CGRPα) locus. We hypothesized that after intercrossing these mice, rapamycin treatment would induce translocation of Venus-FKBP12-Inp54p to the plasma membrane in CGRP+ DRG neurons. In control experiments with cell lines, rapamycin induced translocation of Venus-FKBP12-Inp54p to the plasma membrane, and subsequent depletion of PIP2, as measured with a PIP2 biosensor. However, rapamycin did not induce translocation of Venus-FKBP12-Inp54p to the plasma membrane in FRBPLF-expressing DRG neurons (in vitro or in vivo). Moreover, rapamycin treatment did not alter PIP2-dependent thermosensation in vivo. Instead, rapamycin treatment stabilized FRBPLF in cultured DRG neurons, suggesting that rapamycin promoted dimerization of FRBPLF with endogenous FKBP12.
Conclusions
Taken together, our data indicate that these knockin mice cannot be used to inducibly deplete PIP2 in DRG neurons. Moreover, our data suggest that high levels of endogenous FKBP12 could compete for binding to FRBPLF, hence limiting the use of rapamycin-inducible systems to cells with low levels of endogenous FKBP12.
doi:10.1186/1477-5751-12-13
PMCID: PMC3844522  PMID: 24010830
Phosphatidylinositol 4,5-bisphosphate; PIP2; Rapamycin; Inp54p; FKBP12; Dorsal root ganglia
4.  Photocaged T7 RNA Polymerase for the Light Activation of Transcription and Gene Function in Pro- and Eukaryotic Cells 
A light-activatable bacteriophage T7 RNA polymerase (T7RNAP) has been generated through the site-specific introduction of a photocaged tyrosine residue at the crucial position Tyr639 within the active site of the enzyme. The photocaged tyrosine disrupts polymerase activity by blocking the incoming nucleotide from reaching the active site of the enzyme. However, a brief irradiation with nonphototoxic UV light of 365 nm removes the ortho-nitrobenzyl caging group from Tyr639 and restores the RNA polymerase activity of T7RNAP. The complete orthogonality of T7RNAP to all endogenous RNA polymerases in pro- and eukaryotic systems allowed for the photochemical activation of gene expression in bacterial and mammalian cells. Specifically, E. coli cells were engineered to produce photocaged T7RNAP in the presence of a GFP reporter gene under the control of a T7 promoter. UV irradiation of these cells led to the spatiotemporal activation of GFP expression. In an analogous fashion, caged T7RNAP was transfected into human embryonic kidney (HEK293T) cells. Irradiation with UV light led to the activation of T7RNAP, thereby inducing RNA polymerization and expression of a luciferase reporter gene in tissue culture. The ability to achieve spatiotemporal regulation of orthogonal RNA synthesis enables the precise dissection and manipulation of a wide range of cellular events, including gene function.
doi:10.1002/cbic.201000041
PMCID: PMC3762680  PMID: 20301166
amino acids; caged proteins; light activation; polymerases; RNA
5.  Moving molecules by light; Spatio-temporal manipulation of small GTPase activity at subcellular level and on timescale of seconds in living cells 
Dynamic regulation of the Rho family of small guanosine triphosphatases (GTPases) with great spatiotemporal precision is essential for various cellular functions and events1, 2. Their spatiotemporally dynamic nature has been revealed by visualization of their activity and localization in real time3. In order to gain deeper understanding of their roles in diverse cellular functions at the molecular level, the next step should be perturbation of protein activities at a precise subcellular location and timing.
To achieve this goal, we have developed a method for light-induced, spatio-temporally controlled activation of small GTPases by combining two techniques: (1) rapamycin-induced FKBP-FRB heterodimerization and (2) a photo-caging method of rapamycin. With the use of rapamycin-mediated FKBP-FRB heterodimerization, we have developed a method for rapidly inducible activation or inactivation of small GTPases including Rac4, Cdc424, RhoA4 and Ras5, in which rapamycin induces translocation of FKBP-fused GTPases, or their activators, to the plasma membrane where FRB is anchored. For coupling with this heterodimerization system, we have also developed a photo-caging system of rapamycin analogs. A photo-caged compound is a small molecule whose activity is suppressed with a photocleavable protecting group known as a caging group. To suppress heterodimerization activity completely, we designed a caged rapamycin that is tethered to a macromolecule such that the resulting large complex cannot cross the plasma membrane, leading to virtually no background activity as a chemical dimerizer inside cells6. Figure 1 illustrates a scheme of our system. With the combination of these two systems, we locally recruited a Rac activator to the plasma membrane on a timescale of seconds and achieved light-induced Rac activation at the subcellular level6.
doi:10.3791/3794
PMCID: PMC3460574  PMID: 22433289
Small GTPase; rapamycin; caged compound; spatiotemporal control; heterodimerization; FKBP; FRB; light irradiation
6.  Conditional Transgenesis Using Dimerizable Cre (DiCre) 
PLoS ONE  2007;2(12):e1355.
Cre recombinase is extensively used to engineer the genome of experimental animals. However, its usefulness is still limited by the lack of an efficient temporal control over its activity. We have recently developed a conceptually new approach to regulate Cre recombinase, that we have called Dimerizable Cre or DiCre. It is based on splitting Cre into two inactive moieties and fusing them to FKBP12 (FK506-binding protein) and FRB (binding domain of the FKBP12-rapamycin associated protein), respectively. These latter can be efficiently hetero-dimerized by rapamycin, leading to the reinstatement of Cre activity. We have been able to show, using in vitro approaches, that this ligand-induced dimerization is an efficient way to regulate Cre activity, and presents a low background activity together with a high efficiency of recombination following dimerization. To test the in vivo performance of this system, we have, in the present work, knocked-in DiCre into the Rosa26 locus of mice. To evaluate the performance of the DiCre system, mice have been mated with indicator mice (Z/EG or R26R) and Cre-induced recombination was examined following activation of DiCre by rapamycin during embryonic development or after birth of progenies. No recombination could be observed in the absence of treatment of the animals, indicating a lack of background activity of DiCre in the absence of rapamycin. Postnatal rapamycin treatment (one to five daily injection, 10 mg/kg i.p) induced recombination in a number of different tissues of progenies such as liver, heart, kidney, muscle, etc. On the other hand, recombination was at a very low level following in utero treatment of DiCre×R26R mice. In conclusion, DiCre has indeed the potentiality to be used to establish conditional Cre-deleter mice. An added advantage of this system is that, contrary to other modulatable Cre systems, it offers the possibility of obtaining regulated recombination in a combinatorial manner, i.e. induce recombination at any desired time-point specifically in cells characterized by the simultaneous expression of two different promoters.
doi:10.1371/journal.pone.0001355
PMCID: PMC2131782  PMID: 18159238
7.  Saccharomyces cerevisiae FKBP12 binds Arabidopsis thaliana TOR and its expression in plants leads to rapamycin susceptibility 
BMC Plant Biology  2007;7:26.
Background
The eukaryotic TOR pathway controls translation, growth and the cell cycle in response to environmental signals such as nutrients or growth-stimulating factors. The TOR protein kinase can be inactivated by the antibiotic rapamycin following the formation of a ternary complex between TOR, rapamycin and FKBP12 proteins. The TOR protein is also found in higher plants despite the fact that they are rapamycin insensitive. Previous findings using the yeast two hybrid system suggest that the FKBP12 plant homolog is unable to form a complex with rapamycin and TOR, while the FRB domain of plant TOR is still able to bind to heterologous FKBP12 in the presence of rapamycin. The resistance to rapamycin is therefore limiting the molecular dissection of the TOR pathway in higher plants.
Results
Here we show that none of the FKBPs from the model plant Arabidopsis (AtFKBPs) is able to form a ternary complex with the FRB domain of AtTOR in the presence of rapamycin in a two hybrid system. An antibody has been raised against the AtTOR protein and binding of recombinant yeast ScFKBP12 to native Arabidopsis TOR in the presence of rapamycin was demonstrated in pull-down experiments. Transgenic lines expressing ScFKBP12 were produced and were found to display a rapamycin-dependent reduction of the primary root growth and a lowered accumulation of high molecular weight polysomes.
Conclusion
These results further strengthen the idea that plant resistance to rapamycin evolved as a consequence of mutations in plant FKBP proteins. The production of rapamycin-sensitive plants through the expression of the ScFKBP12 protein illustrates the conservation of the TOR pathway in eukaryotes. Since AtTOR null mutants were found to be embryo lethal [1], transgenic ScFKBP12 plants will provide an useful tool for the post-embryonic study of plant TOR functions. This work also establish for the first time a link between TOR activity and translation in plant cells
doi:10.1186/1471-2229-7-26
PMCID: PMC1903354  PMID: 17543119
8.  Inhibitor-Induced Conformational Stabilization and Structural Alteration of a Mip-Like Peptidyl Prolyl cis-trans Isomerase and Its C-Terminal Domain 
PLoS ONE  2014;9(7):e102891.
FKBP22, an Escherichia coli-encoded PPIase (peptidyl-prolyl cis-trans isomerase) enzyme, shares substantial identity with the Mip-like pathogenic factors, caries two domains, exists as a dimer in solution and binds some immunosuppressive drugs (such as FK506 and rapamycin) using its C-terminal domain (CTD). To understand the effects of these drugs on the structure and stability of the Mip-like proteins, rFKBP22 (a chimeric FKBP22) and CTD+ (a CTD variant) have been studied in the presence and absence of rapamycin using different probes. We demonstrated that rapamycin binding causes minor structural alterations of rFKBP22 and CTD+. Both the proteins (equilibrated with rapamycin) were unfolded via the formation of intermediates in the presence of urea. Further study revealed that thermal unfolding of both rFKBP22 and rapamycin-saturated rFKBP22 occurred by a three-state mechanism with the synthesis of intermediates. Intermediate from the rapamycin-equilibrated rFKBP22 was formed at a comparatively higher temperature. All intermediates carried substantial extents of secondary and tertiary structures. Intermediate resulted from the thermal unfolding of rFKBP22 existed as the dimers in solution, carried an increased extent of hydrophobic surface and possessed relatively higher rapamycin binding activity. Despite the formation of intermediates, both the thermal and urea-induced unfolding reactions were reversible in nature. Unfolding studies also indicated the considerable stabilization of both proteins by rapamycin binding. The data suggest that rFKBP22 or CTD+ could be exploited to screen the rapamycin-like inhibitors in the future.
doi:10.1371/journal.pone.0102891
PMCID: PMC4114562  PMID: 25072141
9.  Association of FKBP51 with Priming of Autophagy Pathways and Mediation of Antidepressant Treatment Response: Evidence in Cells, Mice, and Humans 
PLoS Medicine  2014;11(11):e1001755.
Theo Rein and colleagues examine the role of FKBP51 in the actions of antidepressants, with a particular focus on pathways of autophagy.
Please see later in the article for the Editors' Summary
Background
FK506 binding protein 51 (FKBP51) is an Hsp90 co-chaperone and regulator of the glucocorticoid receptor, and consequently of stress physiology. Clinical studies suggest a genetic link between FKBP51 and antidepressant response in mood disorders; however, the underlying mechanisms remain elusive. The objective of this study was to elucidate the role of FKBP51 in the actions of antidepressants, with a particular focus on pathways of autophagy.
Methods and Findings
Established cell lines, primary neural cells, human blood cells of healthy individuals and patients with depression, and mice were treated with antidepressants. Mice were tested for several neuroendocrine and behavioral parameters. Protein interactions and autophagic pathway activity were mainly evaluated by co-immunoprecipitation and Western blots. We first show that the effects of acute antidepressant treatment on behavior are abolished in FKBP51 knockout (51KO) mice. Autophagic markers, such as the autophagy initiator Beclin1, were increased following acute antidepressant treatment in brains from wild-type, but not 51KO, animals. FKBP51 binds to Beclin1, changes decisive protein interactions and phosphorylation of Beclin1, and triggers autophagic pathways. Antidepressants and FKBP51 exhibited synergistic effects on these pathways. Using chronic social defeat as a depression-relevant stress model in combination with chronic paroxetine (PAR) treatment revealed that the stress response, as well as the effects of antidepressants on behavior and autophagic markers, depends on FKBP51. In human blood cells of healthy individuals, FKBP51 levels correlated with the potential of antidepressants to induce autophagic pathways.
Importantly, the clinical antidepressant response of patients with depression (n = 51) could be predicted by the antidepressant response of autophagic markers in patient-derived peripheral blood lymphocytes cultivated and treated ex vivo (Beclin1/amitriptyline: r = 0.572, p = 0.003; Beclin1/PAR: r = 0.569, p = 0.004; Beclin1/fluoxetine: r = 0.454, p = 0.026; pAkt/amitriptyline: r = −0.416, p = 0.006; pAkt/PAR: r = −0.355, p = 0.021; LC3B-II/PAR: r = 0.453, p = 0.02), as well as by the lymphocytic expression levels of FKBP51 (r = 0.631, p<0.0001), pAkt (r = −0.515, p = 0.003), and Beclin1 (r = 0.521, p = 0.002) at admission. Limitations of the study include the use of male mice only and the relatively low number of patients for protein analyses.
Conclusions
To our knowledge, these findings provide the first evidence for the molecular mechanism of FKBP51 in priming autophagic pathways; this process is linked to the potency of at least some antidepressants. These newly discovered functions of FKBP51 also provide novel predictive markers for treatment outcome, consistent with physiological and potential clinical relevance.
Please see later in the article for the Editors' Summary
Editors' Summary
Background
Everyone feels miserable sometimes, but about one in six people will have an episode of clinical depression during their lifetime. For people who are clinically depressed, overwhelming feelings of sadness, anxiety, and hopelessness can last for months or years. Affected individuals lose interest in activities they used to enjoy, they sometimes have physical symptoms such as disturbed sleep, and they may contemplate suicide. Clinicians diagnose depression and determine its severity using questionnaires (“depression rating scales”) that explore the patient's feelings and symptoms. Mild depression is often treated with talking therapies (psychotherapy) such as cognitive behavioral therapy, which helps people change negative ways of thinking. For more severe depression, patients are also usually prescribed an antidepressant, most commonly a “selective serotonin reuptake inhibitor” such as paroxetine or a tricyclic antidepressant such as amitriptyline.
Why Was This Study Done?
Unfortunately, antidepressants don't work for more than half of patients. Moreover, because it is unclear how antidepressants work, it is not possible to predict which patients will respond to which antidepressants. Thus, matching patient to drug can be a lengthy, sometimes unsuccessful, process. Here, the researchers use several approaches to test the hypothesis that a protein called FK506 binding protein 51 (FKBP51) is involved in the actions of antidepressants and to investigate whether the ability of both FKBP51 and antidepressants to regulate a process called autophagy underlies the impact of FKBP51 on antidepressant responses. FKBP51 is a regulator of stress physiology, which is connected to the development and treatment of depression; genetic studies have suggested a link between FKBP51 expression and the antidepressant response rate. Some antidepressants are known to alter the initial steps in the autophagy pathway, a multistep process that maintains the integrity of cells through regulated degradation and recycling of cellular components; however, the potential synergistic role of FKBP51 and antidepressants in regulating pathways of autophagy are unknown.
What Did the Researchers Do and Find?
The researchers first treated wild-type mice and FKBP51 knockout mice (genetically altered animals that make no FKBP51) with an acute dose of antidepressant and compared their behavior in a forced swim test, an assay that measures the action of antidepressants in mice by determining how long the mice struggle or float inertly when placed in deep water. As expected, acute antidepressant treatment increased the time that wild-type mice spent struggling. However, this effect of antidepressant treatment was greatly attenuated in the FKBP51 knockout mice. Moreover, the levels of several autophagy markers increased in the brains of wild-type mice following antidepressant treatment but not in the brains of FKBP51 knockout mice. Next, using “chronic social defeat stress” to model the “endophenotype” of depression (a combination of physiological, hormonal, and behavioral traits seen in people with depression) in mice, the researchers showed that the stress response and the effect of chronic antidepressants on behavior and on autophagic markers all depend on FKBP51. Using cell-based assays, the researchers showed that antidepressants and FKBP51 had synergistic (interactive) effects on the autophagic pathway and that, in human blood cells, FKBP51 levels correlated with the potential of antidepressants to induce autophagic pathways. Finally, the researchers report that the clinical response to antidepressant treatment in 51 patients with depression was associated with the response of autophagic markers in their peripheral blood lymphocytes to antidepressant treatment in test tubes, and that the expression levels of FKBP51 and autophagy markers in patient lymphocytes at admission were associated with subsequent clinical responses to antidepressants.
What Do These Findings Mean?
These findings suggest that the protein FKBP51 is required for the effects of both acute and chronic treatment with some antidepressants on behavior and on autophagic pathways in mice. These findings also reveal an association between antidepressant treatment responses in patients and both the expression levels of FKBP51 and autophagy markers in lymphocytes at admission and the response of autophagic markers to antidepressant treatment in patient lymphocytes. The accuracy of these findings is limited by the small number of clinical samples available for analysis, by the use of only male mice in the animal experiments, and by the inability of animal models of depression to fully replicate the human condition. Nevertheless, these findings identify the early stages of autophagy as potential targets for the development of new antidepressants and identify several potential biomarkers that might, after further clinical validation, help clinicians predict antidepressant efficacy in patients with depression.
Additional Information
Please access these websites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.1001755.
The US National Institute of Mental Health provides information on all aspects of depression (in English and Spanish), including information on antidepressants
The UK National Health Service Choices website provides detailed information about depression and about antidepressants; it also provides personal stories about depression
The UK charity Mind provides information on depression, including some personal stories about depression
More personal stories about depression are available from healthtalk.org
MedlinePlus provides links to other resources about depression
Wikipedia has a page on autophagy (note that Wikipedia is a free online encyclopedia that anyone can edit; available in several languages)
The patients included in this study were all enrolled in the Munich Antidepressant Response Signature project, which aims to identify gene variants and biomarkers that predict treatment outcomes with antidepressants
doi:10.1371/journal.pmed.1001755
PMCID: PMC4227651  PMID: 25386878
10.  BIMOLECULAR FLUORESCENCE COMPLEMENTATION ANALYSIS OF INDUCIBLE PROTEIN INTERACTIONS: EFFECTS OF FACTORS AFFECTING PROTEIN FOLDING ON FLUORESCENT PROTEIN FRAGMENT ASSOCIATION 
Journal of molecular biology  2009;394(3):391-409.
Bimolecular fluorescence complementation (BiFC) analysis enables visualization of the subcellular locations of protein interactions in living cells. We investigated the temporal resolution and the quantitative accuracy of BiFC analysis using fragments of different fluorescent proteins. We determined the kinetics of BiFC complex formation in response to the rapamycin-inducible interaction between the FK506 binding protein (FKBP) and the FKBP-rapamycin binding domain (FRB). Fragments of YFP fused to FKBP and FRB produced detectable BiFC complex fluorescence 10 minutes after rapamycin addition and a ten-fold increase in the mean fluorescence intensity in 8 hours. The N-terminal fragment of the Venus fluorescent protein fused to FKBP produced constitutive BiFC complexes with several C-terminal fragments fused to FRB. A chimeric N-terminal fragment containing residues from Venus and YFP produced either constitutive or inducible BiFC complexes depending on the temperature at which the cells were cultured. The concentrations of inducers required for half-maximal induction of BiFC complex formation by all fluorescent protein fragments tested were consistent with the affinities of the inducers for unmodified FKBP and FRB. Treatment of the FK506 inhibitor of FKBP-FRB interaction prevented the formation of BiFC complexes by FKBP and FRB fusions, but did not disrupt existing BiFC complexes. Proteins synthesized prior to rapamycin addition formed BiFC complexes with the same efficiency as newly synthesized proteins. Inhibitors of protein synthesis attenuated BiFC complex formation independent of their effects on fusion protein synthesis. The kinetics at which they inhibited BiFC complex formation suggest that they prevented association of the fluorescent protein fragments, but not the slow maturation of BiFC complex fluorescence. Agents that induce the unfolded protein response also reduced formation of BiFC complexes. The effects of these agents were suppressed by cellular adaptation to protein folding stress. In summary, BiFC analysis enables detection of protein interactions within minutes after complex formation in living cells, but does not allow detection of complex dissociation. Conditional BiFC complex formation depends on the folding efficiencies of fluorescent protein fragments and can be affected by the cellular protein folding environment.
doi:10.1016/j.jmb.2009.08.069
PMCID: PMC2862259  PMID: 19733184
Visualization of protein interactions; bimolecular fluorescence complementation (BiFC); fluorescence microscopy and flow cytometry; protein folding; FKBP-FRB association
11.  Rapamycin Antifungal Action Is Mediated via Conserved Complexes with FKBP12 and TOR Kinase Homologs in Cryptococcus neoformans 
Molecular and Cellular Biology  1999;19(6):4101-4112.
Cryptococcus neoformans is a fungal pathogen that causes meningitis in patients immunocompromised by AIDS, chemotherapy, organ transplantation, or high-dose steroids. Current antifungal drug therapies are limited and suffer from toxic side effects and drug resistance. Here, we defined the targets and mechanisms of antifungal action of the immunosuppressant rapamycin in C. neoformans. In the yeast Saccharomyces cerevisiae and in T cells, rapamycin forms complexes with the FKBP12 prolyl isomerase that block cell cycle progression by inhibiting the TOR kinases. We identified the gene encoding a C. neoformans TOR1 homolog. Using a novel two-hybrid screen for rapamycin-dependent TOR-binding proteins, we identified the C. neoformans FKBP12 homolog, encoded by the FRR1 gene. Disruption of the FKBP12 gene conferred rapamycin and FK506 resistance but had no effect on growth, differentiation, or virulence of C. neoformans. Two spontaneous mutations that confer rapamycin resistance alter conserved residues on TOR1 or FKBP12 that are required for FKBP12-rapamycin-TOR1 interactions or FKBP12 stability. Two other spontaneous mutations result from insertion of novel DNA sequences into the FKBP12 gene. Our observations reveal that the antifungal activities of rapamycin and FK506 are mediated via FKBP12 and TOR homologs and that a high proportion of spontaneous mutants in C. neoformans result from insertion of novel DNA sequences, and they suggest that nonimmunosuppressive rapamycin analogs have potential as antifungal agents.
PMCID: PMC104369  PMID: 10330150
12.  Dominant missense mutations in a novel yeast protein related to mammalian phosphatidylinositol 3-kinase and VPS34 abrogate rapamycin cytotoxicity. 
Molecular and Cellular Biology  1993;13(10):6012-6023.
Rapamycin is a macrolide antifungal agent that exhibits potent immunosuppressive properties. In Saccharomyces cerevisiae, rapamycin sensitivity is mediated by a specific cytoplasmic receptor which is a homolog of human FKBP12 (hFKBP12). Deletion of the gene for yeast FKBP12 (RBP1) results in recessive drug resistance, and expression of hFKBP12 restores rapamycin sensitivity. These data support the idea that FKBP12 and rapamycin form a toxic complex that corrupts the function of other cellular proteins. To identify such proteins, we isolated dominant rapamycin-resistant mutants both in wild-type haploid and diploid cells and in haploid rbp1::URA3 cells engineered to express hFKBP12. Genetic analysis indicated that the dominant mutations are nonallelic to mutations in RBP1 and define two genes, designated DRR1 and DRR2 (for dominant rapamycin resistance). Mutant copies of DRR1 and DRR2 were cloned from genomic YCp50 libraries by their ability to confer drug resistance in wild-type cells. DNA sequence analysis of a mutant drr1 allele revealed a long open reading frame predicting a novel 2470-amino-acid protein with several motifs suggesting an involvement in intracellular signal transduction, including a leucine zipper near the N terminus, two putative DNA-binding sequences, and a domain that exhibits significant sequence similarity to the 110-kDa catalytic subunit of both yeast (VPS34) and bovine phosphatidylinositol 3-kinases. Genomic disruption of DRR1 in a mutant haploid strain restored drug sensitivity and demonstrated that the gene encodes a nonessential function. DNA sequence comparison of seven independent drr1dom alleles identified single base pair substitutions in the same codon within the phosphatidylinositol 3-kinase domain, resulting in a change of Ser-1972 to Arg or Asn. We conclude either that DRR1 (alone or in combination with DRR2) acts as a target of FKBP12-rapamycin complexes or that a missense mutation in DRR1 allows it to compensate for the function of the normal drug target.
Images
PMCID: PMC364661  PMID: 8413204
13.  Controlled Dimerization of ErbB Receptors Provides Evidence for Differential Signaling by Homo- and Heterodimers 
Molecular and Cellular Biology  1999;19(10):6845-6857.
The four members of the ErbB family of receptor tyrosine kinases are involved in a complex array of combinatorial interactions involving homo- and heterodimers. Since most cell types express more than one member of the ErbB family, it is difficult to distinguish the biological activities of different homo- and heterodimers. Here we describe a method for inducing homo- or heterodimerization of ErbB receptors by using synthetic ligands without interference from the endogenous receptors. ErbB receptor chimeras containing synthetic ligand binding domains (FK506-binding protein [FKBP] or FKBP-rapamycin-binding domain [FRB]) were homodimerized with the bivalent FKBP ligand AP1510 and heterodimerized with the bifunctional FKBP-FRB ligand rapamycin. AP1510 treatment induced tyrosine phosphorylation of ErbB1 and ErbB2 homodimers and recruitment of Src homology 2 domain-containing proteins (Shc and Grb2). In addition, ErbB1 and ErbB2 homodimers activated downstream signaling pathways leading to Erk2 and Akt phosphorylation. However, only ErbB1 homodimers were internalized upon AP1510 stimulation, and only ErbB1 homodimers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 homodimers were able to associate with and induce phosphorylation of c-Cbl. Cells expressing AP1510-induced ErbB1 homodimers were able to form foci; however, cells expressing ErbB2 homodimers displayed a five- to sevenfold higher focus-forming ability. Using rapamycin-inducible heterodimerization we show that c-Cbl is unable to associate with ErbB1 in a ErbB1-ErbB2 heterodimer most likely because ErbB2 is unable to phosphorylate the c-Cbl binding site on ErbB1. Thus, we demonstrate that ErbB1 and ErbB2 homodimers differ in their abilities to transform fibroblasts and provide evidence for differential signaling by ErbB homodimers and heterodimers. These observations also validate the use of synthetic ligands to study the signaling and biological specificity of selected ErbB dimers in any cell type.
PMCID: PMC84681  PMID: 10490623
14.  A photocleavable rapamycin conjugate for spatiotemporal control of small GTPase activity 
Journal of the American Chemical Society  2010;133(1):10.1021/ja108258d.
We developed a novel method to spatiotemporally control activity of signaling molecules. A newly synthesized photocaged rapamycin derivative induced rapid dimerization of FKBP (FK-506 binding protein) and FRB (FKBP-rapamycin binding protein) upon UV irradiation. With this system and the spatially confined UV-irradiation, we achieved subcellularly localized activation of Rac, a member of small GTPases. Our technique offers a powerful approach to studies of dynamic intracellular signaling events.
doi:10.1021/ja108258d
PMCID: PMC3850177  PMID: 21142151
15.  The Protein Kinase Tor1 Regulates Adhesin Gene Expression in Candida albicans 
PLoS Pathogens  2009;5(2):e1000294.
Eukaryotic cell growth is coordinated in response to nutrient availability, growth factors, and environmental stimuli, enabling cell–cell interactions that promote survival. The rapamycin-sensitive Tor1 protein kinase, which is conserved from yeasts to humans, participates in a signaling pathway central to cellular nutrient responses. To gain insight into Tor-mediated processes in human fungal pathogens, we have characterized Tor signaling in Candida albicans. Global transcriptional profiling revealed evolutionarily conserved roles for Tor1 in regulating the expression of genes involved in nitrogen starvation responses and ribosome biogenesis. Interestingly, we found that in C. albicans Tor1 plays a novel role in regulating the expression of several cell wall and hyphal specific genes, including adhesins and their transcriptional repressors Nrg1 and Tup1. In accord with this transcriptional profile, rapamycin induced extensive cellular aggregation in an adhesin-dependent fashion. Moreover, adhesin gene induction and cellular aggregation of rapamycin-treated cells were strongly dependent on the transactivators Bcr1 and Efg1. These findings support models in which Tor1 negatively controls cellular adhesion by governing the activities of Bcr1 and Efg1. Taken together, these results provide evidence that Tor1-mediated cellular adhesion might be broadly conserved among eukaryotic organisms.
Author Summary
Living organisms have an intrinsic ability to coordinate their growth and proliferation in response to nutrient availability. In organisms ranging from yeasts to humans, the Tor1 signaling pathway responds to nutrient-derived signals and orchestrates cell growth. Accordingly, we find that in the human fungal pathogen Candida albicans, Tor1 signaling also functions to promote growth. We also uncovered a novel role for the Tor1 molecular pathway in promoting hyphal growth of C. albicans on semi-solid surfaces and in controlling cell–cell adherence. Gene expression analysis and genetic manipulations implicate the known cell surface adhesins Als1 and Als3 as mediators of Tor1-regulated cellular adhesion. Further genetic analysis identified the transcriptional regulators Bcr1, Efg1, Nrg1, and Tup1 that together with Tor1 compose a regulatory network governing adhesin gene expression and cellular adhesion. Given that the Tor pathway is the target of several small molecule inhibitors including rapamycin, a versatile pharmacological drug used in medicine, there is considerable interest in Tor signaling pathways and their function. Moreover, given the potent fungicidal activity of rapamycin against C. albicans, novel antifungal therapies remain to be developed, which may also include novel antifungal therapies with less immunosuppressive rapamycin analogs.
doi:10.1371/journal.ppat.1000294
PMCID: PMC2631134  PMID: 19197361
16.  Rapamycin and Less Immunosuppressive Analogs Are Toxic to Candida albicans and Cryptococcus neoformans via FKBP12-Dependent Inhibition of TOR 
Antimicrobial Agents and Chemotherapy  2001;45(11):3162-3170.
Candida albicans and Cryptococcus neoformans cause both superficial and disseminated infections in humans. Current antifungal therapies for deep-seated infections are limited to amphotericin B, flucytosine, and azoles. A limitation is that commonly used azoles are fungistatic in vitro and in vivo. Our studies address the mechanisms of antifungal activity of the immunosuppressive drug rapamycin (sirolimus) and its analogs with decreased immunosuppressive activity. C. albicans rbp1/rbp1 mutant strains lacking a homolog of the FK506-rapamycin target protein FKBP12 were found to be viable and resistant to rapamycin and its analogs. Rapamycin and analogs promoted FKBP12 binding to the wild-type Tor1 kinase but not to a rapamycin-resistant Tor1 mutant kinase (S1972R). FKBP12 and TOR mutations conferred resistance to rapamycin and its analogs in C. albicans, C. neoformans, and Saccharomyces cerevisiae. Our findings demonstrate the antifungal activity of rapamycin and rapamycin analogs is mediated via conserved complexes with FKBP12 and Tor kinase homologs in divergent yeasts. Taken together with our observations that rapamycin and its analogs are fungicidal and that spontaneous drug resistance occurs at a low rate, these mechanistic findings support continued investigation of rapamycin analogs as novel antifungal agents.
doi:10.1128/AAC.45.11.3162-3170.2001
PMCID: PMC90798  PMID: 11600372
17.  Mutually exclusive STAT1 modifications identified by Ubc9/substrate dimerization-dependent SUMOylation 
Nucleic Acids Research  2009;37(4):e30.
Post-translational modifications control the physiological activity of the signal transducer and activator of transcription STAT1. While phosphorylation at tyrosine Y701 is a prerequisite for STAT1 dimerization, its SUMOylation represses the transcriptional activity. Recently, we have demonstrated that SUMOylation at lysine K703 inhibits the phosphorylation of nearby localized Y701 of STAT1. Here, we analysed the influence of phosphorylation of Y701 on SUMOylation of K703 in vivo. For that reason, an Ubc9/substrate dimerization-dependent SUMOylation (USDDS) system was developed, which consists of fusions of the SUMOylation substrate and of the SUMO-conjugating enzyme Ubc9 to the chemically activatable heterodimerization domains FKBP and FRB, respectively. When FKBP fusion proteins of STAT1, p53, CRSP9, FOS, CSNK2B, HES1, TCF21 and MYF6 are coexpressed with Ubc9-FRB, treatment of HEK293 cells with the rapamycin-related dimerizer compound AP21967 induces SUMOylation of these proteins in vivo. For STAT1-FKBP and p53-FKBP we show that this SUMOylation takes place at their specific SUMOylation sites in vivo. Using USDDS, we then demonstrate that STAT1 phosphorylation at Y701 induced by interferon-β treatment inhibits SUMOylation of K703 in vivo. Thus, pY701 and SUMO-K703 of STAT1 represent mutually exclusive modifications, which prevent signal integration at this molecule and probably ensure the existence of differentially modified subpopulations of STAT1 necessary for its regulated nuclear cytoplasmic activation/inactivation cycle.
doi:10.1093/nar/gkp020
PMCID: PMC2651805  PMID: 19174562
18.  Regulation of Cre recombinase by ligand-induced complementation of inactive fragments 
Nucleic Acids Research  2003;31(21):e131.
Cre recombinase is extensively used to engineer the genome of experimental animals. However, its usefulness is still limited by the lack of an efficient temporal control over its activity. To overcome this, we have developed DiCre, a regulatable fragment complementation system for Cre. The enzyme was split into two moieties that were fused to FKBP12 (FK506-binding protein) and FRB (binding domain of the FKBP12–rapamycin-associated protein), respectively. These can be efficiently heterodimerized by rapamycin. Several variants, based on splitting Cre at different sites and using different linker peptides, were tested in an indicator cell line. The fusion proteins, taken separately, had no recombinase activity. Stable transformants, co-expressing complementing fragments based on splitting Cre between Asn59 and Asn60, displayed low background activity affecting 0.05–0.4% of the cells. Rapamycin induced a rapid recombination, reaching 100% by 48–72 h, with an EC50 of 0.02 nM. Thus, ligand-induced dimerization can efficiently regulate Cre, and should be useful to achieve a tight temporal control of its activity, such as in the case of the creation of conditional knock-out animals.
doi:10.1093/nar/gng131
PMCID: PMC275488  PMID: 14576331
19.  InterAKTions with FKBPs - Mutational and Pharmacological Exploration 
PLoS ONE  2013;8(2):e57508.
The FK506-binding protein 51 (FKBP51) is an Hsp90-associated co-chaperone which regulates steroid receptors and kinases. In pancreatic cancer cell lines, FKBP51 was shown to recruit the phosphatase PHLPP to facilitate dephosphorylation of the kinase Akt, which was associated with reduced chemoresistance. Here we show that in addition to FKBP51 several other members of the FKBP family bind directly to Akt. FKBP51 can also form complexes with other AGC kinases and mapping studies revealed that FKBP51 interacts with Akt via multiple domains independent of their activation or phosphorylation status. The FKBP51-Akt1 interaction was not affected by FK506 analogs or Akt active site inhibitors, but was abolished by the allosteric Akt inhibitor VIII. None of the FKBP51 inhibitors affected AktS473 phosphorylation or downstream targets of Akt. In summary, we show that FKBP51 binds to Akt directly as well as via Hsp90. The FKBP51-Akt interaction is sensitive to the conformation of Akt1, but does not depend on the FK506-binding pocket of FKBP51. Therefore, FKBP inhibitors are unlikely to inhibit the Akt-FKBP-PHLPP network.
doi:10.1371/journal.pone.0057508
PMCID: PMC3585324  PMID: 23469007
20.  Novel Fusion Protein Approach for Efficient High-Throughput Screening of Small Molecule–Mediating Protein-Protein Interactions in Cells and Living Animals 
Cancer research  2005;65(16):7413-7420.
Networks of protein interactions execute many different intracellular pathways. Small molecules either synthesized within the cell or obtained from the external environment mediate many of these protein-protein interactions. The study of these small molecule–mediated protein-protein interactions is important in understanding abnormal signal transduction pathways in a variety of disorders, as well as in optimizing the process of drug development and validation. In this study, we evaluated the rapamycin-mediated interaction of the human proteins FK506-binding protein (FKBP12) rapamycin-binding domain (FRB) and FKBP12 by constructing a fusion of these proteins with a split-Renilla luciferase or a split enhanced green fluorescent protein (split-EGFP) such that complementation of the reporter fragments occurs in the presence of rapamycin. Different linker peptides in the fusion protein were evaluated for the efficient maintenance of complemented reporter activity. This system was studied in both cell culture and xenografts in living animals. We found that peptide linkers with two or four EAAAR repeat showed higher protein-protein interaction–mediated signal with lower background signal compared with having no linker or linkers with amino acid sequences GGGGSGGGGS, ACGSLSCGSF, and ACGSLSCGS-FACGSLSCGSF. A 9 ± 2-fold increase in signal intensity both in cell culture and in living mice was seen compared with a system that expresses both reporter fragments and the interacting proteins separately. In this fusion system, rapamycin induced heterodimerization of the FRB and FKBP12 moieties occurred rapidly even at very lower concentrations (0.00001 nmol/L) of rapamycin. For a similar fusion system employing split-EGFP, flow cytometry analysis showed significant level of rapamycin-induced complementation.
doi:10.1158/0008-5472.CAN-05-0588
PMCID: PMC4154795  PMID: 16103094
21.  The Rapamycin-Binding Domain of the Protein Kinase mTOR is a Destabilizing Domain* 
The Journal of biological chemistry  2007;282(18):13395-13401.
Rapamycin is an immunosuppressive drug that binds simultaneously to the 12-kDa FK506- and rapamycin-binding protein (FKBP12, or FKBP) and the FKBP-rapamycin binding domain (FRB) of the mammalian target of rapamycin (mTOR) kinase. The resulting ternary complex has been used to conditionally perturb protein function, and one such method involves perturbation of a protein of interest through its mislocalization. We synthesized two rapamycin derivatives that possess large substituents at the C16 position within the FRB-binding interface, and these derivatives were screened against a library of FRB mutants using a three-hybrid assay in Saccharomyces cerevisiae. Several FRB mutants responded to one of the rapamycin derivatives, and twenty of these mutants were further characterized in mammalian cells. The mutants most responsive to the ligand were fused to yellow fluorescent protein, and fluorescence levels in the presence and absence of the ligand were measured to determine stability of the fusion proteins. Wild-type and mutant FRB domains were expressed at low levels in the absence of the rapamycin derivative, and expression levels rose up to ten-fold upon treatment with ligand. The synthetic rapamycin derivatives were further analyzed using quantitative mass spectrometry, and one of the compounds was found to contain contaminating rapamycin. Furthermore, uncontaminated analogs retain the ability to inhibit mTOR, albeit with diminished potency relative to rapamycin. The ligand-dependent stability displayed by wildtype FRB and FRB mutants as well as the inhibitory potential and purity of the rapamycin derivatives should be considered as potentially confounding experimental variables when using these systems.
doi:10.1074/jbc.M700498200
PMCID: PMC3763840  PMID: 17350953
22.  Active-Site Inhibitors of mTOR Target Rapamycin-Resistant Outputs of mTORC1 and mTORC2 
PLoS Biology  2009;7(2):e1000038.
The mammalian target of rapamycin (mTOR) regulates cell growth and survival by integrating nutrient and hormonal signals. These signaling functions are distributed between at least two distinct mTOR protein complexes: mTORC1 and mTORC2. mTORC1 is sensitive to the selective inhibitor rapamycin and activated by growth factor stimulation via the canonical phosphoinositide 3-kinase (PI3K)→Akt→mTOR pathway. Activated mTORC1 kinase up-regulates protein synthesis by phosphorylating key regulators of mRNA translation. By contrast, mTORC2 is resistant to rapamycin. Genetic studies have suggested that mTORC2 may phosphorylate Akt at S473, one of two phosphorylation sites required for Akt activation; this has been controversial, in part because RNA interference and gene knockouts produce distinct Akt phospho-isoforms. The central role of mTOR in controlling key cellular growth and survival pathways has sparked interest in discovering mTOR inhibitors that bind to the ATP site and therefore target both mTORC2 and mTORC1. We investigated mTOR signaling in cells and animals with two novel and specific mTOR kinase domain inhibitors (TORKinibs). Unlike rapamycin, these TORKinibs (PP242 and PP30) inhibit mTORC2, and we use them to show that pharmacological inhibition of mTOR blocks the phosphorylation of Akt at S473 and prevents its full activation. Furthermore, we show that TORKinibs inhibit proliferation of primary cells more completely than rapamycin. Surprisingly, we find that mTORC2 is not the basis for this enhanced activity, and we show that the TORKinib PP242 is a more effective mTORC1 inhibitor than rapamycin. Importantly, at the molecular level, PP242 inhibits cap-dependent translation under conditions in which rapamycin has no effect. Our findings identify new functional features of mTORC1 that are resistant to rapamycin but are effectively targeted by TORKinibs. These potent new pharmacological agents complement rapamycin in the study of mTOR and its role in normal physiology and human disease.
Author Summary
Growth factor pathways are required for normal development but are often inappropriately activated in many cancers. One growth-factor–sensitive pathway of increasing interest to cancer researchers relies on the mammalian target of rapamycin (mTOR), a kinase that (like all kinases) delivers phosphate groups from ATP to amino acid residues of downstream proteins. TOR proteins were first discovered in yeast as the cellular targets of rapamycin, a small, naturally occurring molecule derived from bacteria that is widely used as an immunosuppressant and more recently in some cancer therapies. The study of TOR proteins has relied heavily on the use of rapamycin, but rapamycin does not directly inhibit TOR kinase activity; rather, rapamycin influences TOR's enzymatic activities by binding to a domain far from the kinase's active site. Some mTOR functions are resistant to rapamycin, as a result of the kinase activity of one kind of multiprotein complex, the mTOR complex 2 (mTORC2), whereas rapamycin-sensitive functions of mTOR are due to the mTOR complex 1 (mTORC1). We have developed new inhibitors of mTOR that bind to the ATP-binding site of mTOR and inhibit the catalytic activity of both mTORC1 and mTORC2 without inhibiting other kinases. Unexpectedly, these inhibitors had profound effects on protein synthesis and cell proliferation due to their inhibition of mTORC1 rather than mTORC2. We found that the phosphorylation of a protein that controls protein synthesis, the mTORC1 substrate 4E binding protein (4EBP) is partially resistant to rapamycin but fully inhibited by our new inhibitors. The finding that 4EBP phosphorylation is resistant to rapamycin suggests that active-site inhibitors may be more effective than rapamycin in the treatment of cancer and may explain why rapamycin is so well tolerated when taken for immunosuppression.
Cells rely on the mammalian target of rapamycin kinase (mTOR) to sense growth factors. Inhibition of all forms of mTOR using newly developed inhibitors of its active site reveals new insights into the function of two mTOR-containing protein complexes and their potential as therapeutic targets.
doi:10.1371/journal.pbio.1000038
PMCID: PMC2637922  PMID: 19209957
23.  Rapamycin inhibits F-actin reorganization and phosphorylation of focal adhesion proteins 
Oncogene  2008;27(37):4998-5010.
An early event of cell migration is characterized as the rapid reorganization of the actin cytoskeleton. Recently we have demonstrated that rapamycin inhibits tumor cell motility. To understand the underlying mechanism, this study was set to determine whether rapamycin inhibition of cell motility is related to its prevention of F-actin reorganization. We found that rapamycin prevented type I insulin-like growth factor (IGF-I)-stimulated F-actin reorganization in human rhabdomyosarcoma (Rh30), Ewing sarcoma (Rh1), glioblastoma (U-373) and prostate carcinoma (PC-3) cells, and concurrently inhibited phosphorylation of focal adhesion proteins, including focal adhesion kinase (FAK), paxillin and p130Cas in the cells. The effect of rapamycin was blocked by expression of a rapamycin-resistant mutant of mTOR (mTORrr), but not a kinase-dead mTORrr. Downregulation of raptor mimicked the effect of rapamycin. Cells infected with a recombinant adenovirus expressing constitutively active and rapamycin-resistant mutant of p70 S6 kinase 1 (S6K1) conferred to resistance to rapamycin. Further, IGF-I failed to stimulate F-actin reorganization and phosphorylation of the focal adhesion proteins in the S6K1-downregulated cells. Expression of constitutively hypophosphoryated eukaryotic initiation factor 4E (eIF4E) binding protein 1 (4E-BP1-5A) inhibited IGF-I-stimulated F-actin reorganization, but did not alter the cellular protein or phosphorylation levels of the focal adhesion proteins. The results suggest that rapamycin inhibits IGF-I-induced F-actin reorganization and phosphorylation of the focal adhesion proteins by disruption of mTOR-raptor complex. Both S6K1 and 4E-BP1 pathways, mediated by the mTOR-raptor complex, are involved in the regulation of IGF-I-stimulated F-actin reorganization, but only the former controls IGF-I-stimulated phosphorylation of the focal adhesion proteins.
doi:10.1038/onc.2008.137
PMCID: PMC2562907  PMID: 18504440
Rapamycin; mammalian target of rapamycin; F-actin; lamellipodia; focal adhesion kinase
24.  FK506 Binding Protein Mediates Glioma Cell Growth and Sensitivity to Rapamycin Treatment by Regulating NF-κB Signaling Pathway1 
Neoplasia (New York, N.Y.)  2008;10(3):235-243.
FK506 binding protein 5 (FKBP5) belongs to a family of immunophilins named for their ability to bind immunosuppressive drugs, also known as peptidyl-prolyl cis-trans isomerases, and also with chaperones to help protein folding. Using glioma cDNA microarray analysis, we found that FKBP5 was overexpressed in glioma tumors. This finding was further validated by real-time reverse transcription-polymerase chain reaction and Western blot analysis. The roles of FKBP5 in glioma cells were then examined. We found that cell growth was suppressed after FKBP5 expression was inhibited by short interfering RNA transfection and enhanced by FKBP5 overexpression. Electrophoretic mobility shift assay showed that nuclear factor-kappa B (NF-κB) and DNA binding was enhanced by FKBP5 overexpression. The expression level of I-kappa B alpha and phosphorylated NF-κB was regulated by the expression of FKBP5. These data suggest that FKBP5 is involved in NF-κB pathway activation in glioma cells. In addition, FKBP5 overexpression in rapamycin-sensitive U87 cells blocked the cells' response to rapamycin treatment, whereas rapamycin-resistant glioma cells, both PTEN-positive and -negative, were synergistically sensitive to rapamycin after FKBP5 was knocked down, suggesting that the FKBP5 regulates glioma cell response to rapamycin treatment. In conclusion, our study demonstrates that FKBP5 plays an important role in glioma growth and chemoresistance through regulating signal transduction of the NF-κB pathway.
PMCID: PMC2259453  PMID: 18320068
25.  The immunosuppressive and toxic effects of FK-506 are mechanistically related: pharmacology of a novel antagonist of FK-506 and rapamycin 
FK-506 inhibits Ca(2+)-dependent transcription of lymphokine genes in T cells, and thereby acts as a powerful immunosuppressant. However, its potential therapeutic applications may be seriously limited by several side effects, including nephrotoxicity and neurotoxicity. At present, it is unclear whether these immunosuppressive and toxic effects result from interference with related biochemical processes. FK-506 is known to interact with FK-binding protein-12 (FKBP-12), an abundant cytosolic protein with cis-trans peptidyl-prolyl isomerase activity (PPIase) activity. Because rapamycin (RAP) similarly binds to FKBP-12, although it acts in a manner different from FK-506, by inhibiting T cell responses to lymphokines, such an interaction with FKBP-12 is not sufficient to mediate immunosuppression. Recently, it was found that the complex of FKBP-12 with FK-506, but not with RAP, inhibits the phosphatase activity of calcineurin. Here, we used L-685,818, the C18- hydroxy, C21-ethyl derivative of FK-506, to explore further the role of FKBP-12 in the immunosuppressive and toxic actions of FK-506. Although L-685,818 bound with high affinity to FKBP-12 and inhibited its PPIase activity, it did not suppress T cell activation, and, when complexed with FKBP-12, did not affect calcineurin phosphatase activity. However, L-685,818 was a potent antagonist of the immunosuppressive activity of both FK-506 and RAP. Moreover, L-685,818 did not induce any toxicity in dogs and rats or in a mouse model of acute FK-506 nephrotoxicity, but it blocked the effect of FK-506 in this model. Therefore, FK-506 toxicity involves the disruption of biochemical mechanisms related to those implicated in T cell activation. Like immunosuppression, this toxicity is not due to the inhibition of the PPIase activity of FKBP- 12, but may be linked to the inhibition of the phosphatase activity of calcineurin by the drug FKBP-12 complex.
PMCID: PMC2119351  PMID: 1380976

Results 1-25 (1118661)